1. Field of the Invention
The present invention relates to electrostatographic fusing apparatus, and more particularly to a heat and pressure roller-type fusing apparatus having a variable shape fuser roller.
2. Description Relative to the Prior Art
Electrostatographic reproduction machines such as copiers and printers, are well known. Such copiers and printers have fusing apparatus, such as heat and pressure roller-type fusing apparatus which are used therein, for fusing toner images. As disclosed for example in U.S. Pat. No. 4,258,648 issued Mar. 31, 1981 to Leising et al., and U.S. Pat. No. 4,594,068 issued Jun. 10, 1986 to Hardutzky et al., the fuser roller of each such fusing apparatus conventionally has a generally cylindrical outer profile or shape for forming a fusing nip with a generally cylindrical pressure roller. As disclosed additionally in the '648 patent, such a generally cylindrical fuser roller includes a metallic core over which is formed at least one elastomeric layer. It is generally also known as pointed out in the '648 patent to apply toner release oil to the surface of such a fuser roller in order to prevent toner image offset onto such fuser roller from a sheet or substrate carrying the image being fused.
Unfortunately, the applied release oil when absorbed into the elastomeric layer of the fuser roller, ordinarily tends to cause such layer to swell undesirably thereby resulting in unacceptable fusing performance. To prevent such undesirable swelling, a very thin outer layer that is impervious to such release oil may for example be added over the elastomeric layer. Unfortunately however, when such a generally cylindrical fuser roller is mounted so as to form a rolling contact fusing nip with the generally cylindrical pressure roller, its high temperature and pressure required for fusing toner images tend to damage such thin outer layer. Such damage ordinarily will allow the surface applied release oil to penetrate through the damaged portion to the inner elastomeric layer thereby causing undesirable swelling. It has been found that such damage to the thin outer layer of the fuser roller occurs frequently within portions of the cylindrical fuser roller that are towards its ends. Particularly such damage occurs within those end portions of the fuser roller which normally are beyond the edges for example of the image carrying sheet or substrate being fed through the fusing nip. Such end portions are of course are not protected from the pressure roller by such sheet or substrate. Accordingly, in such end portions of the fuser roller, high stresses are created between the directly contacting surfaces of the rollers. Such damage also occurs with great frequency at a point of discontinuity which is generally created in the surface of the fuser roller by the edge of the sheet or substrate, particularly under the high temperatures and pressures required for fusing. As a net result, the thin outer layer or coating at such a point, and towards the ends of the fuser roller, tends to fail or be damaged, thus leading to release oil penetration and undesirable swelling of the fuser roller as described above.